Human behavior is intricately dependent on billions of neural connections within the brain. These neural connections are constantly altered and reorganized throughout the life of a human. For example, early in the embryonic and prenatal development of a human being, the cellular morphology of most neurons is characterized by an exuberant, massive outgrowth of dendrites and axons. This tangled network of neural structure allows for synaptic connections to be formed among many neural cells.
As the human being continues to develop, however, extensive reconstruction of the brain and spinal cord occurs. Programmed cell death decreases cell numbers by as much as fifty percent in some brain regions. Axonal and dendritic structures are pruned away and synaptic contacts are removed.
This second, regressive phase of neural development is driven by the interaction of the human being with its environment. For example, as a child acquires new skills, experience, knowledge, and behaviors, unused neural structures are pruned and removed while those that are used are further strengthened. Such activity-dependent changes in neural structure are reflected in tangible transformations visible in the brain's interior terrain.
It is believed that many disorders such as autism, schizophrenia, and depression result from an excess number of neural connections and/or faulty neural connections within the brain. It is likely that these excess and/or faulty neural connections are a result of mistakes made during the developmental establishment of neural connections.